Currently, among secondary batteries, lead storage batteries, which have been most largely produced in the world, are widely used for purposes, such as starting of engines of automobiles, motive power of forklifts and the like, and application for uninterruptible power supplies (UPS) in communication equipment.
With diversified market needs, recent years have seen development of lead storage batteries having improved functions, such as high durability, high capacity, and long-life span.
In particular, long-life battery performance is required for idling stop applications that repeat charge and discharge in a partial state of charge (PSOC) or for DC (Deep Cycle) applications where charge depth is extremely deep.
In relation to the long-life battery performance, a stratification phenomenon may be mentioned as a factor that leads to deterioration of battery performance during long-term use.
Stratification means that the concentration of sulfuric acid used as an electrolytic solution in a batter cell is different between upper and lower layers. Usually, highly concentrated sulfate ions generated due to electrode reaction precipitate. In conventional lead storage batteries that have been used to start engines of automobiles, gas is generated from an electrode surface due to electrode reaction in an overcharged state. Thus, the electrolytic solution has been stirred by a bubbling effect of the gas, whereby the stratification phenomenon has been eliminated and has been no serious concern. On the other hand, in the idling stop applications requiring use in the partial state of charge or DC applications, there occurs no reaction in the overcharged state. Thus, since the electrolytic solution is not stirred by gas, the concentration difference between the upper and lower layers is not eliminated. Near a lower layer electrode having high sulfuric acid concentration, lead sulfate is deposited, which deteriorates life performance.
As means for eliminating the stratification, control valve type lead storage batteries including a fiber-containing separator are known (see Patent Literature 1, Patent Literature 2).
Patent Literature 1 discloses a control valve type lead storage battery using a separator composed of glass fiber, an acid-resistant organic fiber such as acrylic or polyolefin, and silica. The patent literature has reported that the effect of silica on sulfate ion adsorption suppresses stratification, but has neither shown any clear effect of the organic fiber nor described details of the separator structure. In addition, the separator disclosed in the patent literature includes the glass fiber, which may be disadvantageous in that the separator is poor in flexibility and susceptible to shock.
Patent Literature 2 discloses a control valve type lead storage battery using a combination of a first separator formed by an ultrafine glass mat and a second separator formed by a synthetic fiber nonwoven fabric. While it has been reported that the control valve type lead storage battery disclosed in the patent literature has improved cycle life characteristics, neither details of structure of the synthetic fiber nor material thereof has been disclosed.
Accordingly, there is still a need for improvement in the control valve type lead storage batteries disclosed in Patent Literature 1 and Patent Literature 2, from the viewpoint of prolonging the cycle life of battery while ensuring high capacity, high output, and low resistance.